A MishkaCmsCore.Runtime.PageBuilder.Marketplace adapter over the registry this install actually has: the UI-kit bundles sitting on disk.

The Studio's Registry tab was written against a catalogue of things to fetch, and with no remote registry configured its Install button had nothing to install. But an install DOES have a real set of components it could add and has not — a site is normally set up with a subset of a kit, and the other 100 components of that kit are exactly "a package available to install".

So a package here is one uninstalled kit component, and installing it is the same operation the first-run wizard performs: the component plus its dependency closure, through MishkaCmsCore.Install.UiKitCatalog.install_set/2 and UiKitHandler.install_kits!/2. Picking card brings icon with it, because a card that cannot draw its icon is not installed.

What this adapter does NOT do is invent presets. A preset is a page layout and a token set, and nothing on disk supplies one — presets/1 and snippets/1 therefore return nothing, and the Studio says so rather than showing sample cards.